The present invention relates to making a three-dimensional fiber preform.
A particular field of application for the invention is making three-dimensional fiber preforms for fabricating annular parts out of carbon-carbon (C—C) composite material, in particular fabricating brake disks.
Brake disks made of composite material, in particular of composite material having carbon fiber reinforcement and a carbon matrix (carbon/carbon composite) are well known. Fabricating them comprises making an annular fiber preform and densifying it with a matrix.
The fiber preform is usually made by stacking layers or plies of a fiber sheet and by bonding the layers together, typically by needling. The fiber sheet may be linear, in which case the annular shape for the preform is obtained by cutting a disk out from the resulting block that is made up of stacked layers of the fiber sheet. Alternatively, the fiber sheet may be helical with the annular shape of the preform then being obtained directly by stacking and needling layers of the fiber sheet.
The annular fiber preform as obtained in this way is generally densified either by chemical vapor infiltration (CVI), or by using a liquid technique (impregnation with a resin that is a precursor of the matrix, and pyrolyzing the resin).
Another known method of densification to which the invention applies more particularly is film-boiling. This consists in placing the annular fiber preform in a reactor between two spiral inductor coils, the reactor being filled with a carbon precursor that is liquid at ambient temperature, so that the preform and the inductor coils are completely immersed in the liquid. The preform is then heated by electromagnetic coupling until its internal temperature reaches approximately 1000° C., thereby cracking the precursor within the preform and subsequently causing the carbon matrix to be deposited.
The advantage of film-boiling densification compared with CVI densification as conventionally used in the fabrication of brake disks lies in its high speed of densification (about 100 times faster than densification using a gaseous technique). These reaction kinetics are made possible by setting up a steep temperature gradient between the core of the preform and its surface that is cooled by the precursor, which is boiling. Densification thus takes place preferentially in the core of the material, rather than at its surface as happens when performing densification by CVI, thereby making it possible to use conditions that give rise to fast reaction kinetics.
This particular method of densification is intimately associated with the characteristics of the preform itself. In particular, the configuration of the fiber reinforcement, the kind of fibers used, the defects in the preform, etc., all have a significant influence on the temperature profile that is generated during electromagnetic coupling, and thus on the speed with which the preform is densified.